Fig. 1: Electrochemical measurements of OER performances for Co-Cr spinel nanoparticles.

a, b show the linear sweep voltammetry (LSV) curves recorded at a scan rate of 10 mV s^–1 of CoCr₂O₄ nanoparticle after 1, 10, 100, 500, 1000, and 2000 cycles of cyclic voltammetry (CV) measurements and Co₂CrO₄ nanoparticle after 1, 10, 100, 500, and 1000 cycles of CV measurements, respectively; CV profiles recorded at a scan rate of 50 mV s^–1 of c CoCr₂O₄ nanoparticle in 1, 10, 100, 500, 1000, and 2000 cycles and d Co₂CrO₄ nanoparticle in pristine, 10, 100, 500 and 1000 cycles, the corresponding current density changes for both samples as a function of CV during 1000 cycles at 1.65 V vs. RHE are given in (e); f cumulative mass loss comparison of metal dissolution for Co and Cr of CoCr₂O₄ and Co₂CrO₄ from the inductively coupled plasma mass spectrometry (ICP-MS) data. ICP-MS data were measured with selected different CV cycles, i.e. after immersing the electrode into the electrolyte (denoted as 0 cycle), after 5, 10, 20, 100, 500, and 1000 CV cycles without refilling the electrolyte. The compensation resistances are 10.5 ± 0.1 Ω for CoCr₂O₄ and 27.9 ± 0.2 Ω for Co₂CrO₄. All measurements are conducted in 1 M KOH with a pH value of 14.00 ± 0.01, and on 0.196 cm² glassy carbon electrode with a mass loading of ~0.05 mg at room temperature. The rotation speed is 1600 rpm and the compensation resistances (90%) are done automatically by the potentiostat.